Monthly Archives: November 2015

Here’s another example of a greenhouse, this time a huge commercial greenhouse, that uses water as the heating medium.

There are substantial differences between a geothermal water system and this system, namely, the temperature of water they use is very hot relative to a geothermal application and therefore they do not use the system to heat the growing medium directly.

They also use natural gas to heat the water. Natural gas is good because it burns into primarily water vapor and CO2 which they recycle by giving it to plants.

The ultimate source of heat for my greenhouse is the sun. Even if the outside temperature is less than 10C, inside, due to the energy input from the sun, I’m seeing up to 40C on sunny days and almost 30C on overcast days. However, my water reservoir, which I pump through a geothermal bank 6 feet under the ground and through my soil bed, is only seeing a maximum of 15C on those hot days. As nights have been dropping below freezing (-4C), it drops about 5C to 10C when I wake up. That’s not acceptable.

The issue is that the air in the greenhouse can only heat the water if it comes in contact with the reservoir container. Further, waiting for the air to heat up by the sun, and then the air to heat up the water isn’t ideal. Air is a poor conductor of heat and has relatively low energy storage capacity. To improve this situation, we need to increase the surface area of the water system. We could put in a bigger reservoir but space is a scarce resource inside my greenhouse. A larger reservoir will increase the surface area, but only a little bit of that will receive direct photon penetration from the sun. What we really need is a solar heater.

I have a 4 ft x 6 ft area above my components. I could mount a board of some sort up there are coil some water tubes to increase the surface area. I also have 2 old CPU radiators with fans. This will increase the effective surface area of the water’s contact with the air.

Over a couple weekends, I took the 200 ft of black poly tubing and made two spiral loops almost 2 feet in diameter on a 8 ft by 4 ft Styrofoam board I picked up at home depot. In between the two spiral loops, I placed the radiators in series. To hold down the tubes, I used automotive-grade 3M double-sided tape in an “X” pattern. On the top, I used gorilla tape, but this wasn’t effective. Right before installing, I used 10 gauge wire to lock it down in case the 3M tape fails.

I mounted the setup at a 30 degree angle near the top of my greenhouse facing south. This is not ideal. In my area, a 70 degree angle would be better, but space is limited, so this will have to do.

Before I installed it, I tested it on the ground at a 70 degree angle. Over the space of an hour or two, the water heated up from 13C to 19C. It was a sunny day. This is about the same as pouring two good size pots of boiling water inside the reservoir.

I’ll continue tracking the performance, but today I’m optimistic this will help. The running cost is pretty cheap too at only 28 Watts (12V @2.4A max). I’ve coded up an algorithm in my automation controller to only turn on the pump if the air is hotter than the reservoir. This is good for the winter time, but this rule probably won’t work will in the summer. In fact, I may have to add a rule to turn on the solar heating system to help cool the greenhouse. We’ll see.

What about the cost? Well, this addition was relatively cheap. If you want the cheapest solution, this might be it.

It was cold today. It’s been cold all week. The high for today was 7C (about 44F, yes, Tidder, I love you) and the low was 6C. This caused my soil temperature to drop to 11C (52F). This is bad. In addition, it looks like it was very dark most of the day. My artificial lighting accounted for 85% of the total light energy in my greenhouse (how my light algorithm works). The sunlight was only able to warm the greenhouse to 15C.

In a conversation on IRC, a friend, wondered if PEX tubing was very good at thermal conduction (ability to transfer heat energy to another substance). After a bit of searching, I found that PEX has very poor thermal conduction relative to, say, copper. If PEX is 0.4W/(mK), copper is like 28. Big difference. All is not terrible, however, because water is only 0.6W/(mK).

Looking at today’s data, there was a couple degree discrepancy between the reservoir temperature and the soil temperature. I would have thought it would be almost the same. Could thermal conductivity explain the difference?

I conducted an experiment. I boiled some water and poured it into the reservoir. This brought the temperature to about 21C (from about 14C). Over the space of a couple hours, I logged the temperatures from both over the space of 3 hours. Here’s a scatter plot of the reservoir and soil temperatures:

The chart shows a negative correlation between dropping reservoir temperatures falling and rising soil temperatures. We have energy transfer! It also shows about how much pumping time is required to raise the temperature of the soil by a few degrees. This could certainly explain how there could be a difference between the reservoir temperatures and the soil. If the reservoir heats up quickly during peak temperature, it could be several hours before the soil will rise.

During this test, the control soil bed (unregulated) remained around 13.11C and eventually fell to 12.9C after 3hrs. The air temperature dropped from 10.1C to 9.7C. We lost some energy to the air, but not enough to change the air temperature upward. We also likely lost a significant amount of energy to the geothermal bank which as about 4 or 5 time the contact area as the soil bed.

When I started researching about building additional garden beds for food growing I came across the idea of a geothermal air greenhouse while surfing youtube. I had heard of homes being heated/cooled with geothermal and understood it’s efficiency. I started researching the methods used. What struck me as odd is that geothermal homes use liquids, not air like the greenhouses I saw on youtube. Coming from a computer background, I understood the best way to cool a computer, was liquid, not air. Why then use air as the energy transport? I had to do more research.

I ended up watching a thermodynamics lecture series on youtube. This gave me a foundation for more work. Turns out that different substances have different heat properties. For example, to raise the temperature of water by one degree, you’d need about 4 times more energy than what you’d need to raise the temperature of air by one degree. This makes air quicker to heat up, but also quick to cool. Water on the other-hand takes much more energy to heat it up, but retains that energy longer.

I looked at how water is used in radiant heating applications. They use PEX tubing in 9″-spaced serpentine loops in the flooring. Heated water is pumped and the energy transfers bottom-up to the surrounding air. I decided to combine these principles in my greenhouse build. I would dig a hole, lay some PEX tubes through the earth and directly into my garden bed. The garden bed would then “radiant” heat the rest of the greenhouse.

That was the theory then. I was wrong.

Temperatures in my area are now getting really cold at night. Much colder than the 15C that my tomatoes need to be happy. The outside temperatures have gotten as low as 1C. What about inside my greenhouse? Inside, I’ve seen it get to as low as 6C. The radiant heating effect is minimal. So was my experiment a failure? No. Not yet at least.

What I didn’t understand then, but understand more now, is the process of transpiration. Plants move water from the soil up to through the plant to the leaves. I thought I’d be keeping the roots warm, but I’m actually keeping the entire plant warm as it moves the warmer water from the soil up through the plant and out the leaves.

This is why when you want to cool the plant on a hot summer day, you water the soil, not the leaves!

This discovery lead me to the next: greenhouses that use geo air are actually ultimately using water. The plant will store the heat energy in the water inside the plant. It will get it from the soil, which is warmed by air, and it will get it from the ambient air around the plant but ultimately air is cooling/heating water. The above image illustrates how energy is passed around in both situations. There’s not much difference except this: water holds more energy. That means less movement, less digging and lower cost.

There’s still a lot of work to be done before I call the experiment a success. So far the results are positive, but I’ve got several more months of winter to go through.

The particle photon is a cool little wifi device. It’s relatively inexpensive, it connects to the internet, and I’ve been using it for several components in my smart greenhouse project. I needed to add a second irrigation system so I recorded how I built the controller and wrote the basic code to make it “web enabled”.

This video guide is incomplete, but it gives the basics on how to get up and running with your own web enabled internet device.

As a comparison, the Cyber Rain Residential Series 8 zone is $500. It does control 8 zones, and ours can only control 2. It also does not come with the electric valves like ours does. To make a better apples to apples, we’d have to remove the valve from our list and add a bigger relay with more channels. This relay supports 8 channels and is $9. We’ll also need an additional level shifter adding another $1.50. Here’s the new breakdown:

Granted, the software of the Cyber Rain is superior. It supports advanced timers and monitoring. We don’t have that, but we can add all that by writing more code. Let me know if you want me to do a walk through on how to add more smarts to the irrigation controller. I’ve already done it for my setup.